Current gonioscopy lenses are generally of three types. The simplest type comprises a contact lens element having a concave contact lens surface for contacting the cornea of an eye. The lens has a planar mirror (or prism facet) positioned adjacent to and extending anteriorly and laterally outwardly relative to the contact lens surface and the optical axis of the eye. As used herein, mirror or mirror surface is synonymous with prism facet. The mirror is angled such that light rays originating at the periphery of the anterior chamber are reflected outwardly in a direction generally parallel to the optical axis for viewing by the ophthalmologist. More than one mirror may be positioned around the contact lens element so that multiple portions of the eye can be viewed without rotating the lens. Because the image is viewed only through a single mirror, the image viewed by the physician is necessarily inverted. Moreover, it is offset 180° from the actual location of the image being viewed. Because the image is inverted, the lens must be rotated in a direction opposite to the direction one would intuitively move the lens in order to view an adjacent portion of the periphery of the anterior chamber.
A second type of lens commonly called a direct view gonioscopy lens employs a pair of mirror surfaces, one of which is offset from the central axis of the contact lens and the other of which intersects the central axis of the lens. The offset surface originates adjacent a peripheral portion of the contact lens surface and extends anteriorly and laterally therefrom. The second surface extends from a location adjacent and anterior to the bottom portion of the second surface and extends anteriorly from a position on the same side of the central axis as the second surface and intersects the central axis. With this arrangement, the peripheral portion of the anterior chamber can be viewed along the central axis of the lens. With this arrangement, however, only one segment of the periphery of the anterior chamber can be viewed without rotating the lens relative to the eye.
A third type of gonioscopy lens has a first mirror surface on one side of the central axis and a second mirror on the opposite side of the central axis. The two mirror surfaces are juxtaposed on opposite sides of the central axis and are arranged so that light rays from the periphery of the anterior chamber are reflected by the first mirror to the second mirror and then by the second mirror in an anterior direction generally parallel to the central axis. Because the first mirror is positioned on an opposite side of the central axis, the image appears generally to be located above the portion of the anterior chamber being viewed. This mirror arrangement again allows only one portion of the periphery of the anterior chamber to be viewed without rotating the contact lens element.